Method and apparatus for heating a thermionic detector source

Abstract

The bead source for the thermionic detector of a gas chromatograph is heated by the secondary winding of a transformer having a center-tapped primary and operating in the push-pull configuration. The voltage drop across the bead is compared with a preselected reference voltage and an error signal is generated. The error signal is used to control the output of a pulse width modulation control which supplies the transformer primary winding.

I claim: 
     
       1. Apparatus for heating an ion source of a thermionic detector for gas chromatography which comprises, a bead mounted on a platinum wire, said wire being about 0.007 inch in diameter and about 5 mm. in length;   a transformer having a center-tapped primary winding and a secondary winding;   said platinum wire being connected to said secondary winding, means for passing a current from said secondary winding through said bead;   means for establishing a preselected d.c. reference voltage;   means for comparing a voltage drop of said source with said reference voltage including an RMS to DC converter for converting the voltage drop across said source from a.c. to d.c. and producing an output;   means for producing an error signal proportional to the difference between said reference voltage and said RMS to DC Converter output; and   means comprising an about 40 KHz pulse width modulated controller for alternately supplying electrical pulses to either half of said center-tapped primary winding and shaping said pulses by said error signal to minimize said error signal.   
     
     
       2. Apparatus for heating an ion spurce of a thermionic detector for gas chromatography which comprises: a bead mounted on a wire; a toroidal core bead transformer having about a 60 turn center-tapped primary winding and about a two turn floating secondary winding, said wire being connected across said secondary winding to heat said bead, a source of about minus 36 volt d.c. voltage is coupled to one side of said secondary winding to polarize said bead, and a RMS to DC converter coupled through a capacitor to the other side of the secondary winding,   said RMS to DC converter converting a nominal 1 volt RMS signal from said bead to an about 5 volt d.c. signal, a potentiometer having a wiper for providing a desired bead voltage setpoint, an integrating error amplifier for comparing output voltage from said potentiometer and actual bead voltage applied by said RMS to DC converter,   an about 40 Khz pulse width modulated controller circuit being connected to said primary winding, output from said integrating error amplifier being connected to control said pulse width modulated control circuit, operating in a push-pull configuration.   
     
     
       3. Apparatus according to claim 2 wherein said wire is a platinum wire having a diameter of about 0.007 inches and a length of about 5 mm. 
     
     
       4. Apparatus for heating an ion source of a thermionic detector for gas chromatography which comprises: a bead mounted on a wire; a toroidal core bead transformer having an about 60 turn center-tapped primary winding and a two turn floating secondary winding, said wire being connected across said secondary winding to heat said bead, a source of about minus 36 volt d.c. voltage being coupled to one side of said secondary winding to polarize said bead, and an RMS to DC converter coupled through a capacitor to the other side of the secondary winding,   said RMS to DC converter serving to convert a nominal 1 volt RMS signal from said bead to an about 5 volt d.c. signal with a capacitor setting an average time constant and with a resistor and trimming potentiometer setting a gain of about 5 volts, whereby the gain is substantially equivalent to the voltage across the bead, but scaled; a potentiometer having a wiper for providing a desired bead voltage setpoint, an integrating error amplifier for comparing and integrating the difference of the scaled bead voltage from said RMS to DC converter with the setpoint provided by the wiper of said potentiometer, said integrating error amplifier outputting a signal to control an about 40 Khz push-pull configured pulse width modulated controller circuit which is connected to said primary winding to supply an about +1.5 volt peak from the secondary winding to drive said bead, said control signal from said integrating error amplifier serving to control the length of the pulses.